Total obscuring power

Obscuring Power of White Smokes. The total obscuring power, TOP, of a white smoke agent used for screening purposes, is obtained by multiplying the product of volume, in cubic ft or smoke produced per lb of material, and the reciprocal of the smoke layer, in ft, necessary to obscure the filament of a 40-watt Mazda lamp. The TOP for some white smoke agents, at low altitudes where atm constituents are plentiful, is given in Table 1... 

The impact, friction and spark sensitivities of pyrotechnic formulations are assessed by the methods given in Chapter 3. The outlines of methods for the determination of burning rate, luminous intensity, IR intensity, and total obscuring power of smoke are given in this section. 

The measurement of screening performance of smokes is important because smoke screens are one of the countermeasures for IR surveillance systems. The performance of smoke formulations is decided in terms of total obscuring power (TOP), yield factor (Y), mass extinction coefficient (a) followed by calculation of obscuration effectiveness (a. Y. p). These parameters are defined in the following manner. 

TOP total obscuring power (area in sq ft covered by the smoke produced by 1 lb of material)... 

Properties Pale-yellowish, translucent, crystallizable solid of waxy consistency. Sp, gr, 1.82 at 20°. M. P. 44.1°. B. P. 280°. Spontaneously inflammable in air at normal temperature gives off dense white smoke, consisting of phosphorus pentoxide and phosphoric acid. The former has a very high obscuring power. White phosphorus has the greatest total obscuring power (T. 0, P.) of all smoke agents. 

On the basis of total obscuring power (T.O.P.), the smoke agents discussed above, as well as other substances that have been used sina the War for producing smoke, are arranged below in the descending order of their T.O.P. s. 

The surface emitted power or radiated heat flux may be computed from the Stefan-Boltzmann equation. This is very sensitive to the assvuned flame temperature, as radiation varies with temperature to the fourth power (Perry and Green, 1984). Further, the obscuring effect of smoke substantially reduces the total emitted radiation integrated over the whole flame surface. 

Two approaches are available for estimating the surface emitted power the point source and solid plume radiation models. The point source is based on the total combustion energy release rate while the solid plume radiation model uses measured thermal fluxes from pool fires of various materials (compiled in TNO, 1979). Both these methods include smoke absorption of radiated energy (that process converts radiation into convection). Typical measured surface emitted fluxes from pool fires arc given by Raj (1977), Mudan (1984), and Considine (1984). LPG and LNG fires radiate up to 250 kW/m (79,000 Btu/hr-ft ). Upper values for other hydrocarbon pool fires lie in the range 110-170 kW/m (35,000-54,000 Btu/hr- ), but smoke obscuration often reduces this to 20-60 kW/m ( 6300-19,000 Btu/hr-ft ). 

---